1. Field of the Invention
The present invention relates to post-tensioning systems. More particularly, the present invention relates to post-tensioning systems having intermediate anchorages. Furthermore, the present invention relates to sealing devices for preventing liquid intrusion into the exposed sections of tendon in the post-tension system.
2. Description of Related Art
For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
There are many post-tension systems employing intermediate anchorages where the length of the slab is too long to tension with a single anchor. In these systems, the intermediate anchor is interposed between a live end and a dead end anchor. In the construction of such intermediate anchorage systems, the tendon extends for a desired length to the intermediate anchor. A portion of the sheathing is removed in the vicinity of the intermediate anchor. The intermediate anchor is installed onto a form board in accordance with conventional practice. The unsheathed portion of the tendon is received by a tensioning apparatus such that the tendon is stressed in the area between the dead end anchor and the intermediate anchor. After stressing the tendon, concrete is poured over the exterior of the sheathed tendon and over the dead end anchor and intermediate anchor. The remaining portion of the tendon extends from the intermediate anchor to either another intermediate anchorage or to the live end anchor. Intermediate anchorage systems are employed whenever the slab is so long that a single live anchor extending to a single dead end anchor is inadequate. For example, two intermediate anchorages would be used for slabs having a length of approximately 300 feet.
A problem that affects many of the intermediate anchorage systems is the inability to effectively prevent liquid intrusion into the unsheathed portion of the tendon. Normally, the unsheathed portion will extend outwardly, for a distance, from the intermediate anchor in the direction toward the dead end anchor. Additionally, another unsheathed portion will extend outwardly at the intermediate anchor toward the live end anchor. In normal practice with a single live anchor and without intermediate anchors, a liquid-tight tubular member is placed onto an end of the anchor so as to cover the unsheathed portion of the tendon. This is relatively easy to accomplish since the length of the tendon is minimal at the live end. However, it is a considerable burden to attempt to slide such a tubular member along the entire length of the tendon so as to form the liquid-tight seal at the intermediate anchorage. In normal practice, tape, or other corrosion protection materials, are applied to the exposed portion of the tendon adjacent the intermediate anchorage. Extensive practice with this technique has shown that it is generally ineffective for preventing liquid intrusion into the interior of the tendon or into the interior of the intermediate anchorage. As such, a great need has developed in which to protect the exposed areas of the tendon adjacent the intermediate anchorage.
It is an object of the present invention to provide an intermediate anchorage for a post-tension system which facilitates the ability to install the intermediate anchorage system.
It is another object of the present invention to provide an intermediate anchorage for a post-tension system which effectively prevents liquid intrusion into the intermediate anchorage area.
It is another object of the present invention to provide an intermediate anchorage system which avoids the need to thread the intermediate anchor along the great lengths of tendon.
It is a further object of the present invention to provide an intermediate anchorage system for a post-tension system which is easy to install using existing installation techniques.
It is a further object of the present invention to provide an intermediate anchorage for a post-tension system which is easy to use, relatively inexpensive, and simple to manufacture.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.
The present invention is an anchor for an intermediate anchorage system comprising an anchor body having a first wedge-receiving cavity and a second wedge-receiving cavity. The first wedge-receiving cavity has tapered walls narrowing in diameter from a first side to a second side of the anchor body. The second wedge-receiving cavity has tapered walls narrowing in diameter from the second side to the first side of the anchor body. The polymeric encapsulation is in liquid-tight sealing relationship with an exterior surface of the anchor body. The polymeric encapsulation defines a first tubular portion extending outwardly from the first wedge-receiving cavity and axially aligned therewith. The first tubular portion extends outwardly from the second side of the anchor body. The polymeric encapsulation defines a second tubular portion extending outwardly from the second wedge-receiving cavity and axially aligned therewith. The second tubular portion extends outwardly from the first side of the anchor body. The polymeric encapsulation defines a first cap-receiving receptacle extending outwardly of the first wedge-receiving cavity from the first side of the anchor body. The polymeric encapsulation defines a second cap-receiving receptacle extending outwardly of the second wedge-receiving cavity from the second side of the anchor body. A first cap is removably affixed in liquid-tight sealing relationship with the first cap-receiving receptacle. A second cap is removably affixed in liquid-tight sealing relationship with the second cap-receiving receptacle. The first and second caps are formed of a polymeric material.
The present invention is also an intermediate anchorage system for a post-tension system comprising an anchor body having a first cavity and a second cavity in which the first cavity narrows in diameter from a first side to a second side of the anchor body and the second cavity narrows in diameter from the second side to the first side of the anchor body. A first tendon is received within the first wedge-receiving cavity. A first plurality of wedges are received in interference-fit relationship between the first tendon and a wall of the first cavity. The first tendon has an end extending outwardly beyond the second side of the anchor body. A polymeric encapsulation is in liquid-tight sealing relationship with an exterior surface of the anchor body. The first tendon extends outwardly of this polymeric encapsulation. The polymeric encapsulation defines a first tubular portion extending outwardly from the first wedge-receiving cavity and is axially aligned therewith. The first tendon extends through the first tubular portion. The tendon has a sheathed portion and an unsheathed portion. The first plurality of wedges are in interference-fit relationship with the unsheathed portion of the tendon. The first tubular portion is in liquid-tight sealing relationship with the sheathed portion of the tendon. The polymeric encapsulation also defines a first cap-receiving receptacle extending outwardly from the first cavity on the first side of the anchor body. A first cap is removably affixed to the first cap-receiving receptacle. The tendon has an end positioned interior of the first cap.
In this intermediate anchorage system, a second tendon is received within the second cavity. A second plurality of wedges are received in interference-fit relationship between the second tendon and a wall of the second cavity. The second tendon extends outwardly beyond the first side of the anchor body. The polymeric encapsulation also defines a second tubular portion extending outwardly from the second cavity and axially aligned therewith. The second tendon extends through this second tubular portion. The polymeric encapsulation further defines a second cap-receiving receptacle extending outwardly from the second cavity from the second side of the anchor body. A second cap is removably affixed to the second cap-receiving receptacle. The tendon has an end positioned interior of this second cap.
The present invention is also a method of post-tensioning an intermediate anchorage in a post-tension system comprising the steps of: (1) forming a dead-end anchorage having a first tendon affixed thereto; (2) forming an anchor body having a first cavity and a second cavity which narrow in opposite directions; (3) affixing an end of a second tendon within the second cavity such that the second tendon extends outwardly from a first side of the anchor body; (4) positioning the dead-end anchorage and the anchor body within a form; (5) installing an end of the first tendon within the first cavity of the anchor body; (6) solidifying concrete around the dead-end anchorage and the anchor body within the form; (7) tensioning the first tendon from the first side of the anchor body; and (8) affixing the first tendon in a tensioned state within the first cavity.
The method of the present invention also includes positioning a live-end anchorage within another portion of the form. The live-end anchorage has a cavity formed therein. The second tendon is extended through this form so that the end of the second tendon is received within the cavity of the live-end anchorage. Concrete is solidified around the second tendon and the live-end anchorage. The end of the second tendon is tensioned from the live-end anchorage. The second tendon is then affixed in a tensioned state within the cavity of the live-end anchorage.